Low thermal expansion NI-base superalloy

ABSTRACT

A low thermal expansion Ni-base superalloy contains, by weight % (hereinafter the same as long as not particularly defined), C: 0.15% or less; Si: 1% or less; Mn: 1% or less; Cr: 5 to 20%; at least one of Mo, W and Re of Mo+½ (W+Re) of 10 to 25%; Al: 0.2 to 2%; Ti: 0.5 to 4.5%; Fe of 10% or less; at least one of B: 0.02% and Zr: 0.2% or less; a remainder of Ni and inevitable impurities; wherein the atomic % of Al+Ti is 2.5 to 7.0.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a low thermal expansion Ni superalloy, and more particularly to a low thermal expansion Ni superalloy having high strength and excellent corrosion-resistance and oxidation-resistance.

[0003] 2. Description of the Related Art

[0004] In recent years, the bolt material for high temperature which is used for a pressure vessel member which is heated to the high temperature, such as a chamber of a steam turbine and gas turbine is made of 12 Cr ferritic steel (containing C: 0.12%, Si: 0.04%, Mn: 0.7%, P: 0.1%, Ni: 0.4%, Cr: 10.5%, Mo: 0.5%, Cu: 0.03%, V: 0.2%, W: 1.7%, Nb: 0.1% and Fe: remaining percent) or austenitic heat-resistant alloy (Nimonic alloy 80A including Cr: 10.5%, Mn: 0.4%, Al: 1.4%, Ti: 2.4%, Si: 0.3%, C: 0.06%, Zr: 0.06%, B: 0.003%, Ni: remaining percent, and Refrataloy 26 including Cr: 18%, Co: 20%, Mo: 3%, Ti: 2.6%, Fe: 16%, C: 0.05%, Ni: remaining percent).

[0005] In recent years, in order to improve the thermal efficiency of a steam turbine, the steam temperature has been further raised so that the high temperature bolt has been used under more severe conditions. Where each of the materials described above is used for the high temperature bolt under such a severe condition, 12 Cr ferritic steel is low in cost and excellent in production. However, if the steam temperature becomes higher than at present, the material is low in the strength at the high temperature. On the other hand, austenitic heat-resistant alloy is more excellent in the corrosion-resistance and oxidation-resistance than the 12 Cr ferritic steel, and high in the high temperature strength. However, because it has a higher linear expansion coefficient than that of 12 Cr ferritic steel, it may produce leakage of steam due to insufficient tightening of the bolt, and generate thermal fatigue. Therefore, austenitic heat-resistance ally is also problematic as a material used at higher temperatures.

[0006] JP-A-9-157779 discloses a low thermal expansion Ni-base super heat-resistant alloy with excellent corrosion-resistance and oxidation-resistance containing, by weight %, C of 0.2% or less, Si of 1% or less, Mn of 1% or less, Cr of 10 to 24%, one or more kinds of Mo and W of Mo+½ W of 5 to 17%, Al of 0.5 to 2%, Ti of 1 to 3%, Fe of 10% or less, B of 0.02 or less and Zr of 0.2% or less, and as necessary Co of 5% or less and Nb of 1.0% or less and remainder of Ni and inevitable impurities. JP-A-8-85838 also discloses a similar alloy.

[0007] A previously known example of alloys having a low linear expansion coefficient is Inconel 783 of an Invar alloy (containing Cr: 3.21%, Mn: 0.08%, Al: 5.4%, Ti: 0.2%, Si: 0.07%, C: 0.03%, B: 0.003%, Fe: 24.5%, Ni: 28.2% and Co: 35.3% . . . Comparative Example No. 2) which has been developed as the material for a jet engine. This alloy has a low linear expansion coefficient in a ferromagnetic state with the Curie point adjusted in the balance of Fe—Ni—Co. However, this alloy does not have corrosion-resistance enough to be used for the steam turbine.

SUMMARY OF THE INVENTION

[0008] It is an object of the present invention to provide a low expansion Ni-base superalloy having a linear expansion coefficient approximately equal to 12 Cr ferritic steel, and high-temperature strength and corrosion/oxidation-resistance approximately equal to the above austenite heat-resistant alloy.

[0009] In order to solve the above problems, the inventors of the invention have eagerly investigated the low linear expansion Ni-base superalloy. As a result, the inventors found that as regards Mo, W and Re, when the value represented by Mo+½ (W+Re) is 10 or more, the target thermal expansion coefficient can be obtained; in order to increase the thermal expansion coefficient in this case, Cr should be 20% or less; the thermal expansion coefficient further lowers where the value of Mo+½ (W+Re) exceeds 17 and Cr is lower than 10%; and even if Cr is lower than that of a conventional Ni-base heat-resistant alloy, a problem of steam oxidation does not occur, and have accomplished the invention on the basis of these findings.

[0010] A low thermal expansion Ni-base superalloy of the present invention comprises, by weight % (hereinafter the same as long as not particularly defined), C: 0.15% or less; Si: 1% or less; Mn: 1% or less; Cr: 5 to 20%; at least one of Mo, W and Re of Mo+½ (W+Re) of 10 to 25%; Al: 0.2 to 2%; Ti: 0.5 to 4.5%; Fe of 10% or less; at least one of B: 0.02% and Zr: 0.2% or less; a remainder of Ni and inevitable impurities; wherein the atomic % of Al+Ti is 2.5 to 7.0.

[0011] In the low thermal expansion Ni-base superalloy, it is preferable that the amount of Cr is from 5 to 10 (exclusive) %; wherein the amount of at least one of Mo, W and Re of Mo+½ (W+Re) is from 17 (exclusive) to 25%; the amount of Al is from 0.2 to 0.4 (exclusive) %; and/or the amount of Ti is from 3.5 (exclusive) to 4.5%.

[0012] The low thermal expansion Ni-base superalloy may further comprises at least one of Nb and Ta in Nb+½ Ta: 1.5% or less; wherein the atomic % of Al+Ti+Nb+Ta is 2.5 to 7.0.

[0013] In the low thermal expansion Ni-base superalloy, a part of Ni may be replaced by Co of 5% or less. In the low thermal expansion Ni-base superalloy, it is preferable that an average expansion coefficient at a temperature from room temperature to 700° C. is 14.0×10⁻⁶/° C. or less.

DETAILED DESCRIPTION OF THE INVENTION

[0014] An explanation will be given of the reason why the composition of the components is defined as described above.

[0015] C: 0.15% or Less

[0016] Element C is contained to create carbide in combination with Ti, Nb, Cr and Mo, enhance the high temperature strength and prevent the size of the crystal grain from increasing. The contents of C exceeding 0.15% decreases the property of hot working so that it is 0.15% or less and preferably 0.10% or less.

[0017] Si: 1% or Less

[0018] Element Si is added as deoxidant and contained to increase the oxidation resistance. The contents of Si exceeding 1% reduces ductility so that it is 1% or less, preferably 0.5% or less.

[0019] Mn: 1% or Less

[0020] Element Mn is added as deoxidant like Si. The contents of Mn exceeding 1% deteriorates the high temperature oxidation characteristic and also promotes precipitation of η phase (Ni, Ti) spoiling the ductility so that it is 1% or less, preferably 0.5% or less.

[0021] Cr: 5 to 20%

[0022] Cr is contained to improve the high temperature resistance and corrosion resistance through solid solution in the austenite phase. In order to maintain the sufficient high temperature oxidation resistance and corrosion resistance, although more contents of Cr is desired, it increases the thermal expansion coefficient so that it desired to be less from the standpoint of view of the thermal expansion.

[0023] In order to obtain a target thermal expansion coefficient in the vicinity of 650 to 700° C. which is a using temperature intented by the invention, the contents of Cr of 5 to 20% is desired. In order to obtain a lower thermal expansion coefficient, the contents of Cr is preferably 5 to 15%, and further lower thermal expansion coefficient. the contents of Cr is preferably 5 to 10 (exclusive) %.

[0024] Mo+½ (W+Re): 10 to 25%

[0025] Elements Mo, W and Re are contained in order to increase the high temperature strength through strengthening of solid solution in the austenite phase and reduce the thermal expansion coefficient. In order to obtain the thermal expansion coefficient intended by the invention, the total of one or more kinds of Mo+½ (W+Re) is at least 10% or more. The total of them exceeding 25% reduces the property of hot working and precipitates the embrittling phase to reduce the ductility so that the contents of Mo+½ (W+Re) is set at 10 to 25%. In order to obtain a lower thermal expansion coefficient, the contents of Mo+½ (W+Re) is preferably 17 (exclusive) to 25%.

[0026] Ti: 0.5 to 4.5%

[0027] Element Ti is contained to strengthen the γ′ phase formed in combination with Ni, reduce the thermal expansion coefficient and promote the effect of aging precipitation in the γ′ phase. In order to provide such an effect, the contents of 0.5% or more must be contained. However, the contents of 4.5% or more precipitates the η phase (Ni, Ti) of the embrittling phase to reduce ductility so that it is set at 0.5 to 4.5%. In order to obtain the sufficient strength and low thermal expansion coefficient at the using temperature of 700° C. intended by the invention, the contents of Ti preferably exceeds 3.5% and 4.5% or less.

[0028] Al: 0.2 to 2.0%

[0029] Element Al is the most important element to create the γ′ phase in combination with Ni and strengthen by it's the precipitation. The contents of less than 0.2% provides insufficient precipitation of the γ′ phase. Where a large quantity of Ti, Nb and Ta makes the γ′ phase unstable and precipitates η phase and phase to cause embrittlement. The contents of 2.0% or more deteriorates the property of hot working and makes it impossible to forge a component. Therefore, the contents is set at 0.2 to 2.0% and preferably 0.2 to 0.4 (exclusive) %.

[0030] Fe: 10% or Less

[0031] Element Fe is an impurities contained when inexpensive scrap or inexpensive mother alloy containing W, Mo, etc. is used in order to reduce the cost of the alloy. The element Fe decrease the high temperature strength and increase the thermal expansion coefficient. Although the less content thereof is preferred, the content of 10% or less slightly influences the high temperature strength so that it is set at 10% or less. Preferably, it is 5% or less, and more preferably, it is 2% or less.

[0032] B: 0.02% or less, Zr: 0.2% or Less

[0033] Elements B and Zr segregates in a crystal grain boundary to increase the creep strength. In addition, the element B can suppress the precipitation of η-phase in the alloy containing a larger quantity of Ti. These elements B and Zr are contained to provide such an effect. Excessive content of these elements deteriorates the property of hot working and excessive Zr spoils the creep characteristic. For these reasons, the content of B is set at 0.02% and that of Zr is set at 0.2% or less.

[0034] Co: 5% or Less

[0035] Element Co is contained to increase the high temperature strength in solid solution in the alloy. However, the effect is relatively low as compared with the other elements and expensive. For this reason, the content thereof is set at 5% or less.

[0036] Nb+½ Ta: 1.5% or Less

[0037] Elements Nb and Ta can form the γ′ phase (Ni₃ (Al, Nb, Ta) which is a precipitation strengthening phase of Ni-base superalloy and have the effects of strengthening the γ′ phase and preventing the coarsening of γ′ phase. These elements are contained to provide such an effect. Excessive content thereof precipitates the δ phase (Ni₃ (Nb, Ta) to lower ductility. For this reason, the content of Nb+½ Ta is set at 1.5%. The desired range is 1.0% or less.

[0038] Ni: Remainder

[0039] Element Ni is an main element to create austenite which serves as matrix, and can increase heat-resistance and corrosion-resistance. In addition, Ni forms the γ′ phase which is a precipitation strengthening phase.

[0040] Al+Ti: 2.5 to 7.0% by atomic %, Al+Ti+Nb+Ta: 2.5 to 7.0% by Atomic %

[0041] Elements Al, Ti, Nb and Ta are constituents of the γ′ phase. Therefore, where there is sufficient quantity of Ni, the volume fraction of the precipitated γ′ phase is proportional to the total of the atomic percent of these elements. Further, the high temperature strength is proportional to the volume fraction of the γ′ phase so that it increases with the total of these elements. Therefore, the content thereof of 2.5% or more is required to acquire the sufficient strength. However, the contents thereof exceeding 7.0% excessively increases the volume fraction of the γ′ phase to deteriorate the property of hot working remarkably. For this reason, the content thereof is set at 2.5 to 7.0% by atomic %, preferably 3.5 to 6.0%.

[0042] Other Elements

[0043] As regards elements Mg, Ca, P, S and Cu, the property of the low thermal expansion Ni-base superalloy according to the invention will not be deteriorated as long as Mg: 0.03% or less, Ca: 0.03% or less, P: 0.05% or less, S: 0.001% or less, and Cu: 2% or less.

[0044] The low thermal expansion Ni-base superalloy according to the invention can be prepared by the same method as a conventional method for preparing Ni-base superalloy. The heat treatment, after solid-solution heat treatment not less than 950° C., is effective in a single step aging (700 to 850° C.) and a two-step aging (first step: 800 to 900° C., second step: 700 to 800° C.).

EXAMPLES

[0045] Various examples of the present invention will be explained below.

[0046] The alloy having the compositions as shown in Table 1 was molten in a vacuum induction furnace having a capacity of 50 kg and its ingot having 50 kg was cast. The surface of an ingot of the ingot was cut away and the ingot was heat-treated for 15 hr at 1150° C. as a homogenizing treatment. Thereafter, the ingot was forged into rods each having 60 mm square. The forged rods were heated for 2 hr at 1100° C., and thereafter water-cooled for its solid solution. The rods were hardening-treatment aged for 16 hr at 750° C. Sample pieces cut out from the rods were subjected to various tests. Thus, the test results as shown in Table 2 were obtained.

[0047] As regards the thermal expansion coefficient, using quartz as a standard sample, the average thermal expansion coefficient from room temperature to 70° C. was measured by a dilatometer available from RIGAKU DENSI CO. LTD. The measurement was carried out under the condition of a temperature rising speed of 5° C./min on the basis of a differential dilatometry. The sample used has a size of φ5×L19.

[0048] The high temperature tensile test was carried out for a tensile specimen with ridges having a parallel portion of 6 mm in diameter at 700° C. on the basis of the JIS high temperature tensile test method.

[0049] The creep rupture test was carried out for a specimen with a parallel portion having 6.4 mm in diameter at 700° C. under load stress of 343 MPa.

[0050] The steam oxidation test which is problematic in a steam turbine was carried out for the specimen having a width of 10 mm, length of 10 mm and thickness of 5 mm for 100 hr at 600° C., thereby measured the weight gain of oxidation after the test. The test was carried out in an environment of atmospheric pressure, water-vapor concentration of 83% and a water-steam flow rate of 7.43 l/s. TABLE 1 Mo + (W + Al + Ti + No. C Si Mn Ni Fe Co Cr Re W Wo Ta Nb Al Ti Zr B Re)/2 Nb + Ti 1 0.11 0.19 0.42 * 6.91 7.75 2.08 5.03 10.51 0.80 2.30 0.006 14.07 4.8 2 0.02 0.08 0.09 * 0.41 6.14 5.10 13.61 0.81 2.35 0.004 16.18 5.0 3 0.06 0.11 0.12 * 0.61 6.93 5.11 11.93 0.26 2.88 0.06 0.011 14.49 4.3 4 0.06 0.15 0.25 * 0.47 12.01 19.25 1.81 0.91 0.008 19.25 5.2 5 0.02 0.31 0.21 * 0.96 14.11 17.08 0.70 2.40 0.04 0.004 17.08 4.6 6 0.05 0.09 0.12 * 0.21 18.12 17.21 0.51 1.98 0.03 0.003 17.21 3.6 7 0.06 0.11 0.12 * 0.48 10.12 4.92 17.51 0.48 2.42 0.06 0.011 19.97 4.3 8 0.04 0.08 0.09 * 1.02 11.91 19.07 0.61 3.21 0.03 0.005 19.07 5.5 9 0.06 0.09 0.25 * 0.62 10.51 12.13 8.24 0.39 2.48 0.008 14.31 4.3 10 0.03 0.25 0.36 * 0.43 3.91 9.52 4.17 13.23 0.6 0.90 2.20 0.05 0.004 15.32 5.3 11 0.03 0.06 0.06 * 0.54 1.92 7.16 4.96 15.04 1.11 1.65 0.03 0.005 17.52 4.7 12 0.05 0.06 0.11 * 0.36 2.14 10.12 4.12 19.18 0.7 1.10 1.61 0.03 0.004 21.24 5.2 13 0.05 0.09 0.09 * 0.41 4.42 11.91 4.96 13.94 0.6 0.8 0.39 2.79 0.05 0.006 16.42 5.3 14 0.04 0.21 0.42 * 0.97 7.82 4.21 19.11 1.20 1.60 0.05 0.003 21.22 4.9 15 0.04 0.05 0.08 * 0.51 9.03 1.11 3.90 18.67 0.80 2.30 0.02 0.006 21.18 4.9 16 0.03 0.07 0.10 * 0.34 7.11 4.08 20.12 1.05 1.71 0.04 0.003 22.16 4.7 17 0.02 0.09 0.09 * 0.51 9.01 4.90 17.01 0.45 2.01 0.05 0.007 19.46 3.7 18 0.04 0.09 0.08 * 0.84 8.17 4.01 14.06 0.75 3.51 0.04 0.004 16.07 6.3 19 0.02 0.09 0.11 * 0.21 9.01 5.10 17.12 0.5 0.5 0.51 2.41 0.03 0.003 19.67 4.9 20 0.03 0.10 0.08 * 0.32 9.10 4.95 16.51 0.5 0.49 2.51 0.03 0.002 18.99 4.8 21 0.02 0.12 0.13 * 0.24 12.13 19.13 0.5 0.38 2.49 0.03 0.003 19.13 4.4 22 0.03 0.11 0.21 * 0.12 9.13 5.01 16.91 0.38 3.61 0.03 0.002 19.42 5.7 23 0.03 0.09 0.12 * 0.24 9.23 17.12 0.5 0.35 3.54 0.03 0.002 17.12 5.7 C1 0.12 0.04 0.72 * 10.51 1.72 0.51 0.1 V: 0.2 1.37 C2 0.04 0.11 0.09 * 0.91 19.11 1.41 2.46 0.004 5.8 C3 0.04 0.21 0.32 * 0.41 18.92 18.12 2.86 0.21 2.81 0.003 2.86 3.8 C4 0.03 0.07 0.08 * 24.51 35.30 3.21 3 5.39 0.21 0.003 13.0 C5 0.03 0.09 0.07 * 41.80 13.02 4.7 0.03 1.48 0.003 4.8 C6 0.04 0.09 0.08 * 0.23 9.12 13.10 7.92 2.41 2.51 0.04 0.003 14.47 9.0 C7 0.03 0.09 0.12 * 0.35 11.23 13.70 7.50 1.51 3.24 0.05 0.004 14.35 7.9 C8 0.04 0.09 0.12 * 0.87 19.12 1.41 8.12 0.42 2.51 0.05 0.004 8.83 4.0 C9 0.03 0.08 0.11 * 0.41 14.12 8.20 23.5 0.56 2.51 0.04 0.003 27.62 4.8 C10 0.04 0.11 0.12 * 0.21 10.12 4.11 15.86 0.36 1.12 0.05 0.003 17.92 2.3

[0051] TABLE 2 Room temperature to 700° C. 600° C. × 500 hr average 700° C./ weight thermal 700° C. 343 MPa gain of expansion Tensile creep steam coefficient strength rupture oxidation (X10⁻⁶/° C.) (MPa) life (hr) (mg/cm²)  1 13.4 900 1513 0.17  2 12.9 915 1625 0.16  3 13.1 933 1012 0.16  4 13.2 928 1131 0.09  5 13.8 996 1025 0.08  6 13.4 958 894 0.05  7 12.7 1001 1341 0.16  8 13.0 1109 981 0.15  9 12.9 896 1532 0.21 10 13.3 931 835 0.15 11 12.9 890 1019 0.16 12 12.8 996 1216 0.11 13 12.7 1014 2531 0.11 14 12.7 970 1083 0.12 15 12.7 1017 899 0.11 16 12.5 980 1395 0.13 17 12.8 930 791 0.14 18 12.9 1069 2482 0.16 19 12.4 1007 2780 0.13 20 12.8 999 1987 0.15 21 13.1 1014 2108 0.11 22 12.5 1118 2880 0.16 23 13.1 1078 2541 0.14 C1 12.4 178 3.19 C2 14.5 771 1011 0.17 C3 16.1 774 1697 0.16 C4 13.0 922 422 1.90 C5 11.3 956 398 2.38 C6 C7 C8 14.1 866 768 0.12 C9 C10 13.0 641 501 0.18

[0052] As understood from the results shown in Table 2, all the samples according to the invention have the average thermal expansion coefficient of 14.0×10⁻⁶/° C. or less at the temperature from room temperature to 700° C., and the tensile strength of 890 to 1118 MPa at 700° C. They have the creep rupture life of 791 to 2880 hr, and the weight gain of steam oxidation of 0.05 to 0.21 mg/cm².

[0053] On the other hand, comparative example No. 1, which is 12 Cr ferritic steel, has a low average thermal expansion coefficient of 12.4×10⁻⁶/° C. However, it's high temperature tensile strength is lower than the samples according to the invention. Comparative examples Nos. 2 and 3, which are Nimonic 80A and Refractaloy 26 known as a high temperature bolt material. These alloys have average thermal expansion coefficients of 14.5×10⁻⁶/° C. and 16.1×10⁻⁶/° C., respectively which are higher than those of the samples according to the invention. Comparative examples Nos. 4 and 5, which are Inconel 783 and Incoloy 909, have average thermal expansion coefficients which are equal or lower than those of the samples according to the invention, but have worse steam oxidation characteristics than those according to the invention.

[0054] Comparative example No. 6, which has an Al content exceeding the upper limit of the invention and a total quantity of Al+Ti exceeding the upper limit of the invention, produced a crack in the material by water-cooling during the solid solution heat treatment. Comparative -example No. 7, which has a total quantity of Al+Ti exceeding the upper limit of the invention, like the comparative example No. 6, produced a crack in the material by water-cooling during the solid solution heat treatment, and hence could not evaluated thereafter.

[0055] Comparative example No. 8, which is an alloy containing more Cr and a smaller value of Mo+½ (W+Re) than those of the samples according to the invention, has a larger average thermal expansion coefficient of 14.1×10⁻⁶/° C. than those of the samples according to the invention.

[0056] Comparative example No. 9, which is an alloy having a larger value of Mo+½ (W+Re), has worse forgeability. This alloy produced a crack during the forging and could not evaluated thereafter.

[0057] Comparative example No. 10, which is lower in the total of Al+Ti than in the invention and insufficient in the precipitation amount of γ′ phase, has a smaller high-temperature strength than those of the samples according to the invention.

[0058] The low thermal expansion Ni-base superalloy according to the invention, which has the compositions as shown, has the average thermal expansion coefficient of 14.0×10⁻⁶/° C. which is approximately equal to that of 12 Cr ferritic steel, and also has the creep rupture life of 791 to 2880 hr and weight gain of steam oxidation of 0.05 to0.21 mg/cm². Thus, the Ni-base superalloy according to the invention has an excellent effects of the high temperature strength and corrosion/oxidation resistance where are approximately equal to those of the austenite heat-resistant alloy.

[0059] The low thermal expansion Ni-base superalloy can be applied to the bolt, blade and disk of a steam turbine, gas turbine and jet engine, and also applied to a boiler tube of a heating machine and pressurizing machine, thereby giving an excellent effect of improving the reliability of a thermal power plant. 

What is claimed is:
 1. A low thermal expansion Ni-base superalloy comprising, by weight % (hereinafter the same as long as not particularly defined), C: 0.15% or less; Si: 1% or less; Mn: 1% or less; Cr: 5 to 20%; at least one of Mo, W and Re of Mo+½ (W+Re) of 10 to 25%; Al: 0.2 to 2%; Ti: 0.5 to 4.5%; Fe of 10% or less; at least one of B: 0.02% and Zr: 0.2% or less; a remainder of Ni and inevitable impurities; wherein the atomic % of Al+Ti is 2.5 to 7.0.
 2. The low thermal expansion Ni-base superalloy according to claim 1, wherein the amount of Cr is from 5 to 10 (exclusive) %.
 3. The low thermal expansion Ni-base superalloy according to claim 2, further comprising at least one of Nb and Ta in Nb+½ Ta: 1.5% or less; wherein the atomic % of Al+Ti+Nb+Ta is 2.5 to 7.0.
 4. The low thermal expansion Ni-base superalloy according to claim 1, wherein the amount of at least one of Mo, W and Re of Mo+½ (W+Re) is from 17 (exclusive) to 25%.
 5. Thelow thermal expansion Ni-base superalloy according to claim 4, further comprising at least one of Nb and Ta in Nb+½ Ta: 1.5% or less; wherein the atomic % of Al+Ti+Nb+Ta is 2.5 to 7.0.
 6. The low thermal expansion Ni-base superalloy according to claim 1, wherein the amount of Al is from 0.2 to 0.4 (exclusive) %.
 7. The low thermal expansion Ni-base superalloy according to claim 6, further comprising at least one of Nb and Ta in Nb+½ Ta: 1.5% or less; wherein the atomic % of Al+Ti+Nb+Ta is 2.5 to 7.0.
 8. The low thermal expansion Ni-base superalloy according to claim 1, wherein the amount of Ti is from 3.5 (exclusive) to 4.5%.
 9. The low thermal expansion Ni-base superalloy according to claim 8, further comprising at least one of Nb and Ta in Nb+½ Ta: 1.5% or less; wherein the atomic % of Al+Ti+Nb+Ta is 2.5 to 7.0.
 10. The low thermal expansion Ni-base superalloy according to any one of claims 1 to 9, wherein a part of Ni is replaced by Co of 5% or less.
 11. The low thermal expansion Ni-base superalloy according to claim 1, wherein an average expansion coefficient at a temperature from room temperature to 700° C. is 14.0×10⁻⁶/° C. or less. 